Inductively coupled plasma (ICP) sources have advantages over other types of plasma sources when used with a focusing column to form a focused beam of charged particles, i.e., ions or electrons. The inductively coupled plasma source, such as the one described in U.S. Pat. No. 7,241,361, which is assigned to the assignee of the present invention, is capable of providing charged particles within a narrow energy range, which allows the particles to be focused to a small spot. ICP sources include a radio frequency (RF) antenna typically wrapped around a ceramic plasma chamber. The RF antenna provides energy to maintain the plasma within the chamber.
The energy of ions used for ion beam processes is typically between 5 keV and 50 keV, and most typically about 30 keV. The electron energy varies between about 1 keV to 5 keV for a scanning electron microscope system to several hundred thousand electron volts for a transmission electron microscope system. The sample in a charged particle system is typically maintained at ground potential, with the source maintained at a large electrical potential, either positive or negative, depending on the particles used to form the beam. For the safety of operating personnel, it is necessary to electrically isolate the high voltage components.
It is usually not possible to ignite a plasma in an ICP source by injection of the normal power level of the RF power used to drive the coil of the ICP source. This is because, in the absence of any initial ionization in the source chamber, the induced electrical field is usually not high enough to break down the gas atoms or molecules to create sufficient initial free charges. To generate this initial ionization, typically a high voltage pulse is required. In the prior art, a high voltage pulse to ignite the plasma in the ICP ion source has been initiated by contacting a Tesla coil to an electrode which is itself in direct electrical contact with the plasma chamber. The high voltage pulse induced by the Tesla coil then initiates a plasma which is subsequently sustained by the RF power from the ICP power supply. This plasma ignition method necessarily requires that there be direct electrical contact between some exterior electrode on the system and the interior of the vacuum system where the plasma is to be ignited. However, when the plasma is biased to high voltage for use as the source in a charged particle beam system, such a direct electrical contact would present serious safety concerns since the external connection would float up to the plasma potential at high voltage. Thus, it is generally not possible to provide such a direct external electrical contact to the plasma in an ICP ion source which is biased to high voltage for use as the source in an ion beam system. This electrical isolation of the high voltage plasma thus creates a problem for igniting the plasma in an ICP source used to generate a charged particle beam.